[sdiy] Adapting exponential convertor for single supply

[Tom Wiltshire]

The rest of the circuit is a 12dB/oct VCF (and as Paul P said, there are thousands of LM13700 designs that run on a single supply, so there’s no problem there) modulated by a PIC-based LFO - a sort of auto-wah pedal thing. There’s a simple “Cutoff Freq” control which sets the basic position from which the modulation is applied.

The problem I’m having is that the frequency control and the modulation response are linear, so it’s rather touchy at the lower end, and then not very responsive at the higher end. So I thought “Ah! No problem! I’ll just stick some simple exponential convertor design in there” and started hunting for something that might be suitable, or which could be bashed into shape.

As such, it doesn’t have to have brilliant temperature stability or accurate tracking. It just needs to turn a linear response into a roughly log/expo one to improve the pot feel and the modulation response. That diff pair circuit looked like it might be pretty much what I was after, although it’s all wired for dual supplies. But there’s no *requirement* to use a dual supply for a diff pair, so that should be easy to fix.

Perhaps I should re-phrase my problem: What’s the simplest exponential convertor that can accept a 0-5V input, runs from a 0-9V single supply, and produces a current suitable for a LM13700?

Thanks,
Tom

PS, Paula: Yeah, I did wonder about sticking an extra look-up table in the code to do a expo conversion before I output the LFO, but I don’t have any pins left to add the Cutoff control on the digital side, and I wouldn’t be able to maintain the full mod depth across the range.

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 27 Jul 2017, at 06:38, Paul Perry <pfperry at melbpc.org.au> wrote:
> 
> There are thousands of single supply guitar effect pedals using LM13700 &
> LM13600 op amps, on 9V DC supplies, so it's certainly possible to use them.
> 
> The problem is connecting up with the rest of the circuitry - in audio gear the
> coupling caps solve most problems. But with synth stuff it's usually
> direct coupled & there will be tears. TI actually make a chip described as a "rail splitter"
> for this application - not sure if it really is extra useful or just a way to get a few extra $$.
> http://www.ti.com/product/TLE2426
> 
> paul perry Melbourne Australia
> 
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